DIY Surface Mount Assembly

[rowan.bradley]

I am about to try to assemble my first SMD board. I'm going to use the skillet method (i.e. heat the board up to whatever temperature is necessary by laying it on a hot metal surface). My board has nothing on the bottom surface so will lie flat on the hot metal plate. Some questions about solder paste and flux.

I see that there are various grades of solder paste, with different degrees of activity. For normal work, which level works best? Some of my components may have been in stock for a while. The PCBs are fairly new.

Some solder pastes contain lead, others don't. Which works best? I'm more concerned about solderability than about complying with ROHS (these boards are not for sale).

What temperature will I need to heat the board to for good reflow to occur? I have a couple of thermocouples attached to my hot metal plate, so I can control the temperature of the plate (which may of course not be the same as the temperature of the PCB itself) fairly closely.

When I buy a 25g solder paste syringe, do I use this as it is, or do I need finer needles to apply the paste to the pads of fine pitch IC footprints? If I need needles, what size do they need to be, and where do I buy them?

Do I need to use flux as well as solder paste? If so, what type?

Do I need to clean the boards after reflow? If so, what sort of solvent do I use?

I can't imagine that it is really possible to clean properly underneath an IC in a (for example) TQFN16 package, since it has quite a large area in the middle that must be (after soldering) more or less in contact with the board surface (at least, not with a saucer of solvent and a brush). How does one deal with this situation, if using a flux that requires cleaning?

Thanks for your help - Rowan

 

[BobK]

So many questions!

I don't know about different grades of solder paste.

I doubt that you could use a finer needle. On the syringes that I have used, it is very difficult to squeeze the solder paste out through the needle they come with, making one's hand very sore.

You do not need flux. Solder paste is small solder particles suspended in flux.

I don't think you need cleaning since the flux is confined to the areas actually soldered.

Now some question for you:

Why the skillet method? I have done SMT boards completely with soldering iron and, after a little practice it is actually easier than through hole in some ways.

Are you making multiples of the same boards? If so, a stencil is the way to go. I have not used this method, but others here have, perhaps they will comment.

Bob

 

[rowan.bradley]

So many questions!

I don't know about different grades of solder paste.

I doubt that you could use a finer needle. On the syringes that I have used, it is very difficult to squeeze the solder paste out through the needle they come with, making one's hand very sore.

You do not need flux. Solder paste is small solder particles suspended in flux.

I don't think you need cleaning since the flux is confined to the areas actually soldered.

Now some question for you:

Why the skillet method? I have done SMT boards completely with soldering iron and, after a little practice it is actually easier than through hole in some ways.

Are you making multiples of the same boards? If so, a stencil is the way to go. I have not used this method, but others here have, perhaps they will comment.

Bob

Biob,

Thanks for your advice. I did wonder whether it was possible to force solder paste through a fine needle. But I have some quite fine pitch IC pads to apply solder to. I will have to practice and find a technique that works.

I guess the need for cleaning depends on how aggressive a flux one is using. I have ordered a syringe of paste, and we will see how it works.

I chose the skillet method, because after reading around it seemed likely to be more reliable than soldering iron. Presumably it's not possible to use a soldering iron for chips that have pads that are underneath, and are not exposed at the edge (.e.g. BGA parts)? I don't have any such parts at present, apart from some parts that have a large square pad in the middle which I am told to earth. These could not be soldered with an iron, I don't think. I'm not actually using a skillet, since these seem next to impossible to buy outside USA. I'm using a plate of 5mm thick aluminium, on top of an electric hotplate, with a perspex lid. The plate is fitted with two thermocouples, so I can continually read its temperature. I have not at this stage tried to automate the temperature regulation, although this is an obvious enhancement. We will see shortly how well this works, and if it doesn't work well I may well resort to the soldering iron method. At present I'm just making prototypes. I think if I got into anything like production, I would try to find someone (maybe one of these chinese companies) who will assemble the boards for me.

Thanks - Rowan

 

[HellasTechn]

I think you need a matrix to apply solder paste on the pcb. if not then you may create short circuits.

 

[rowan.bradley]

What is a "matrix"? Do you mean a stencil?

I think I will have a go just using a solder paste syringe, with a magnifier, and see how difficult it is, and whether I get shorts or dry joints or whatever. If it proves too difficult, then I will try the stencil approach. I have a friend who was running a company in the USA, and trying to build SMD boards right in the early days of SMD (in the 1980s I suppose). I remember his stories about how with some of his early boards you just bent the board slightly, and all the chips would fly off. I think the problem then turned out to be that the components had been in storage for too long, and had presumably got tarnished. So it amazes me that nowadays an amateur can do an adequate job with minimal equipment in his kitchen.

Rowan

 

[hevans1944]

In the next few months I will attempt to modify a Black and Decker toaster oven for re-flow oven soldering. Plenty of advice on the Web on how to do that. The stencil is a very good idea and you can order them inexpensively. If you are going to dispense solder paste manually, I would go with Bob's suggestion and hand-solder after attaching the big ICs with heat-sink pads underneath using your hot-plate method. Make sure the solder paste is fresh. Most I have seen for sale have a recommended shelf life. Don't know how this stuff ages in the syringe, but purchase and use immediately. Check the expiry date and send it back if "use by" date has been exceeded. Your miles (or kilometers) may differ.

 

[BobK]

You don't feed the solder from the syringe.

For parts with only a few leads, you squeeze it out onto the pads, place the part, then just touch each pad with the tip of the soldering iron.

For ICs with many pins, you do drag soldering, using regular solder. Basically, you flux all the pins, and place the part. Then you tack down diagonal corners with the soldering iron. Then, for each row of pins, you drag a bead of solder across them, not worrying that you are bridging all the pins. Finally, you use solder braid to sop up most of the solder, removing the bridges and leaving a minimal amount to connect the pins. Look for a video on YouTube, it is really quite easy to do.

Here is an example of a board I did this way:

https://www.electronicspoint.com/projectlogs/my-wearable-device.8/

For pads on the bottom, you can put a few vias under the pad, then, after the rest of the chip is soldered, heat these from the other side to melt the solder paste you put under the pad.

Bob

 

[HellasTechn]

What is a "matrix"? Do you mean a stencil?

Yes a stencil.

 

[davenn]

I think you need a matrix to apply solder paste on the pcb. if not then you may create short circuits.

In my experience, not necessary, as the paste contracts to the pads when heated

have replaced dozens of these chips ... very fine pin pitch

 

[rowan.bradley]

Bob said:

>Basically, you flux all the pins

What sort of flux?

Yes, I know about the solderirng iron method (in theory - i've never tried it - as I say, this is my first SMD board).

The via idea under heatsink pads sounds a good idea. Unfortunately too late for me, since I've already ordered the boards. When the boards arrive, I will try the various methods suggested and try to find something that works. I will report back then.

Thanks for all the advice - Rowan

 

[davenn]

Bob said:

>Basically, you flux all the pins

What sort of flux?

The solder paste

as he said earlier ....

You do not need flux. Solder paste is small solder particles suspended in flux.

Dave

 

[rowan.bradley]

"flux all the pins" sounds a strange way of describing applying solder paste.

Rowan

 

[davenn]

"flux all the pins" sounds a strange way of describing applying solder paste.

Rowan

just a little variation
could have been worded a little better when aimed at newcomers
the rest of us knew what was meant


Dave

 

[HellasTechn]

In my experience, not necessary, as the paste contracts to the pads when heated

have replaced dozens of these chips ... very fine pin pitch

I can take your word for that since I have never tried that. what I have practiced with chips like the one in the picture is to apply a layer of solder with the solder iron, shorting all pins together and then with a flat tip solder iron to remove the excessive solder and leave the pins soldered on their pads. Some times solder sucker is needed if there is too much solder.

 

[HellasTechn]

I have many boards with chips like that. i will use some to practice and let you know of my experience

 

[HellasTechn]

In my experience, not necessary, as the paste contracts to the pads when heated

do you use a hot air station for that ? if yes then what temp setting do you use ? I know that from station to station this setting cant be the same but just as reference.

 

[BobK]

"flux all the pins" sounds a strange way of describing applying solder paste.

Rowan

It was not. You do not use solder paste for drag soldering, you use wire solder. My description does leave something to be desired. The flux should be applied after tacking down the corner pins. I use a liquid flux pen, putting a goodly amount over the pins and pads. This allows the solder bead that you dragging to flow over everything and stick everywhere.

Bob